1. Field of Invention
The present invention relates to methods and sensing devices for automotive transmissions and, more particularly, to non-contacting magnetoelastic torque sensors for providing a measure of the torque transmitted radially in a transmission converter drive plate or similar disk-shaped member.
2. Description of the Related Art
In the control of systems having rotating drive shafts, torque and speed are fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable, and inexpensive manner has long been a primary objective of such control system designs.
Previously, torque measurement was accomplished using contact-type sensors directly attached to a shaft. One such sensor is a “strain gauge” type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by torque-induced strain is measured by a bridge circuit or other well known means. However, contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are expensive and are thus commercially impractical for competitive use in many applications, such as automotive steering systems, for which torque sensors are sought.
Subsequently, non-contact torque sensors of the magnetostrictive type were developed for use with rotating shafts. For example, U.S. Pat. No. 4,896,544 to Garshelis, which is incorporated herein by reference, discloses a sensor comprising a torque-carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically-directed residual stress-induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torqued member, differences in the response of the two bands to forces applied to the torque-carrying member. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the external magnetic fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used can create practical problems in applications where space is at a premium. Also, such sensors may be impractically expensive for use on highly cost-competitive devices, such as in automotive applications.
Torque transducers based on measuring the field arising from the torque induced tilting of initially circumferential remanent magnetizations have been developed which, preferably, utilize a thin wall ring (“collar”) serving as the field generating element. See, for example, U.S. Pat. Nos. 5,351,555 and 5,520,059 to Garshelis, which are incorporated herein by reference. Tensile “hoop” stress in the ring, associated with the means of its attachment to the shaft carrying the torque being measured establishes a dominant, circumferentially directed, uniaxial anisotropy. Upon the application of torsional stress to the shaft, the magnetization reorients and becomes increasingly helical as torsional stress increases. The helical magnetization resulting from torsion has both a radial component and an axial component, the magnitude of the axial component depending entirely on the degree of torsion. One or more magnetic field vector sensors may be used to sense the magnitude and polarity of the field arising, as a result of the applied torque, in the space above the magnetically conditioned regions on a shaft, and provide a signal output reflecting the magnitude and direction of the torque. Inasmuch as the peak allowable torque in a ring sensor is limited by slippage at the ring/shaft interface, concerns have been expressed regarding distortion arising from slippage at the ring/shaft interface under conditions of torque overload. This, together with the need for multiple parts of different materials to minimize the adverse effects of parasitic fields, has encouraged the investigation of alternative constructions.
Magnetoelastic torque transducers have been developed in which the active, torque sensing region is formed directly on the shaft itself, rather than on a separate ferromagnetic element which then has to be affixed to the shaft. See, for example, U.S. Pat. No. 6,047,605 to Garshelis, which is incorporated herein by reference. In one form of these so-called “collarless” transducers, the magnetoelastically active region is polarized in a single circumferential direction and itself possesses sufficient magnetic anisotropy to return the magnetization in the region, following the application of torque to the member, to the single circumferential direction when the applied torque is reduced to zero. The torqued shaft is desirably formed of a polycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90-degree quadrant symmetrically disposed around the direction of magnetic polarization and have a coercivity sufficiently high that the transducing region field does not create parasitic magnetic fields in proximate regions of the shaft of sufficient strength to destroy the usefulness, for torque sensing purposes, of the net magnetic field seen by the magnetic field sensor. In particularly preferred forms of such transducers the shaft is formed of a randomly oriented, polycrystalline material having cubic symmetry and the coercivity is greater than 15 Oersted (Oe), desirably greater than 20 Oe and, preferably, greater than 35 Oe.
More recently, non-contacting magnetoelastic torque sensors have been developed that provide signals indicative of the torque transmitted between radially separated locations of disk-shaped members. U.S. Pat. No. 6,513,395 to Jones, which is incorporated herein by reference, describes a torque sensor that includes a disk-shaped member having a magnetoelastically active region that is polarized in a single circumferential direction. In that patent, a magnetic field sensor is mounted proximate to the active region, the sensor sensing the magnitude of a magnetic field resulting from a torque transferred from a shaft to the disk-shaped member, and the sensor outputting a signal in response thereto. Such a configuration may be susceptible to compassing as discussed below. That patent also describes a disk having dual circumferentially and oppositely polarized regions, with two sensors positioned along the same radial line, their sensitive directions oriented radially and oppositely to permit common mode field cancellation. This placement of sensors, however, has the undesired result in which the sensors pick up magnetic field signals that do not change linearly in response to a change in torque applied to the disk.
Other prior art describes a torque sensor that includes a disk-shaped member having a region in which annular magnetically conditioned regions are separated from one another and spaced in a radial direction. It is believed, however, that a torque sensor having a gap between magnetically conditioned regions may exhibit a large rotational signal uniformity (RSU) signal due to random magnetic leakage fields between the two annular magnetically conditioned regions. Ideally, a torque sensor will exhibit a zero RSU signal, which is defined as no variation in signal output during the rotation of a member when no torque, or a constant torque, is applied to the rotating member. In actual practice however, due to deficiencies in the surface preparation and magnetization processes, noticeable RSU signals are detected. Furthermore, a torque sensor having a disk-shaped member with a gap between magnetically conditioned regions requires additional space, which is not desirable in applications in which the disk has a limited amount of flat surface available for magnetically conditioned regions.
Because magnetic fields, in the context of their measurement, are fungible, the sensors taught by the above and other prior art may be susceptible to other magnetic fields of exterior origin. In particular, the earth's magnetic field will cause a phenomenon known as “compassing,” in which the measured field is the sum of the torque induced magnetic field and the earth's magnetic field. Within the context of this disclosure, the term “compassing” shall be used to describe any error resulting from the earth's magnetic field.
Magnetic fields of external origin can emanate from both far field and near field sources. A far field source, such as the earth with its magnetic field, generally has the same effect on each magnetic field sensor in a torque sensing device having multiple magnetic field sensors. Near field sources, such as permanent magnets, magnetized wrenches, motors, solenoids, etc., may create magnetic fields having significant local gradients, thus having significantly different effects on the different magnetic field sensors in a torque sensing device having multiple magnetic field sensors.
U.S. Pat. No. 5,520,059 to Garshelis addresses the compassing issue with respect to far field sources. In that patent, a shaft is described having two axially distinct magnetoelastically active regions, polarized in opposite circumferential directions, with magnetic field sensors having opposite axial polarities positioned proximate to the active regions and providing output signals in response to a torque applied to the shaft. By summing the outputs of the magnetic field sensors, all common mode external magnetic fields, i.e. far fields, are canceled. In applications employing such a scheme, the oppositely polarized sensors should be placed as close to each other as possible to preserve the efficiency of the common mode rejection scheme. Sensors that are spaced from one another exhibit reduced common mode rejection efficiency, as the earth's magnetic field may be significantly distorted around ferromagnetic parts in and around the torque sensor.
U.S. Pat. App. Pub. No. 2009/0230953 to Lee, which is incorporated herein by reference, describes a torque sensing device designed to cancel near field magnetic noise from external sources without canceling a torque-induced magnetic field. That reference describes a torque sensor including three sets of magnetic field sensors, axially spaced proximate to a shaft, the shaft having a magnetoelastically active region that is polarized in a circumferential direction. Signals received by each of the magnetic field sensors are adjusted to compensate for the effects of near field sources.
In torque sensing devices having ferromagnetic members with annular magnetoelastically active regions, it is desirable for a magnetic field sensor placed proximate to the magnetoelastically active region to pick up a signal that accurately represents the torque applied to the member, regardless of the angular distance between the magnetic field sensor and a radius of the member. Torque sensing devices that demonstrate this characteristic are said to demonstrate improved rotational signal uniformity (RSU). Non-uniformities in the depth, width, or magnetic field strength, about an annular magnetoelastically active region may lead to noticeable RSU signals and, hence, inaccurate torque measurements Improved RSU performance, and a decreased hysteresis effect, may also be achieved by subjecting the ferromagnetic member to an appropriate surface hardness process, as is known in the art, prior to magnetization. Lee, for example describes a torque sensing device designed to exhibit improved RSU performance by incorporating a plurality of angularly and axially spaced magnetic field sensors placed proximate to a circumferential surface of a rotatable shaft.
The torque sensing devices described in the prior art are not specially configured for measuring the torque transmitted between a shaft and a radially separated portion of a disk-shaped member, while demonstrating improved RSU performance and reducing detrimental effects caused by compassing. Accordingly, there exists a need for such a device.